KR20170059006A - Use, stabilization and carbonization of polyacrylonitrile/carbon composite fibers - Google Patents

Abstract

In the method for producing carbon fibers, carbon nanotubes (CNTs) were mixed with a solution containing polyacrylonitrile (PAN) to form a CNT / PAN mixture. From this mixture one or more PAN / CNT fibers were formed. A predetermined first current was applied to the PAN / CNT fibers until the PAN / CNT fibers became stabilized PAN / CNT fibers. A heatable fabric comprising a plurality of fibers each having an axis. The plurality of fibers each comprise polyacrylonitrile and carbon nanotubes dispersed at predetermined weight percentages in the polyacrylonitrile and aligned along the axis of the plurality of fibers. The plurality of fibers are woven into the fabric. The current source is configured to be energized through a plurality of fibers to generate heat in the fibers.

Description

USE, STABILIZATION AND CARBONIZATION OF POLYACRYLONITRILE / CARBON COMPOSITE FIBERS, USE OF POLYACRYLONITRILE /

Cross-reference of related application (s)

The present application claims priority from U.S. Provisional Patent Application Serial No. 61 / 901,519, filed November 8, 2013, the disclosure of which is incorporated herein by reference. The present application claims priority from U.S. Provisional Patent Application Serial No. 61 / 903,048, filed November 12, 2013, the disclosure of which is incorporated herein by reference. The present application claims priority from U.S. Provisional Patent Application Serial No. 62 / 002,761, filed May 23, 2014, the disclosure of which is incorporated herein by reference. The present application claims priority from U.S. Provisional Patent Application Serial No. 62 / 004,053, filed May 28, 2014, the disclosure of which is incorporated herein by reference.

STATEMENT OF GOVERNMENT RIGHTS

The invention was made with Government support under convention number W911NF-10-1-0098 signed by the Army. The US government has certain rights in the invention.

Field of the Invention

The present invention relates to carbon and polymeric fibers, and more particularly to spinning fibers comprising carbon nanotubes.

Carbon fibers have been used in many applications ranging from aircraft structural components requiring light weight and high strength to tennis racquets. Most of the carbon fibers produced today are stabilized and made from carbonized polyacrylonitrile (PAN) precursor fibers.

PAN is a synthetic, semi-crystalline organic polymer resin with a linear formula (C ₃ H ₃ N) _n . Most of the polyacrylonitrile resins are copolymers containing acrylonitrile-based monomers. PAN is often spun into fibers as precursors of high-quality carbon fibers through solution spinning processes. In fact, PAN is used as a precursor of 90% of carbon fiber production.

In one example of a spinning process, the PAN powder is dissolved in an organic solvent to form a solution. The solution is spun through the orifice of the spinnerette and the resulting fibers are drawn into an environment that solidifies as fibers.

To be used in numerous carbon fiber applications, the fibers must be carbonized in a process that removes non-carbon elements from the fibers. Typically, carbonization is carried out in a high temperature in an inert environment. However, the heat required to carbonize the PAN fibers generally destroys the fibers before they are carbonized. Therefore, PAN fibers need to be stabilized before carbonization. Stabilization is usually carried out in air.

Stabilization of the PAN fibers results in a ladder-like structure. The stabilization process usually entails heating the fibers in a furnace in an oxygen enriched environment. Once stabilized, the fibers are treated in an inert environment of high temperature to remove non-carbon atoms to form carbon fibers.

Conventional systems typically expose PAN fibers to heat in the oven. The stabilization process consumes a considerable amount of energy and takes considerable time, both adding carbon fiber costs. Generally, fabrication of polyacrylonitrile (PAN) based carbon fibers requires large furnaces to stabilize and carbonize precursor PAN fibers. These fibers are usually stabilized in the air within the temperature range of 180 ° C to 350 ° C and carbonized in an inert environment of 350 ° C to 1700 ° C. The stabilization time may generally vary from 1 to 3 hours.

Also, using an oven-based stabilization method may result in improper stabilization of the fibers. Since all heat from the oven is radiated from the outside of the fiber into the interior of the fiber, there can be different levels of stabilization across the cross section of the fiber: the outer shell of the fiber can be overstabilized but the center of the fiber can be less stabilized have. This can lead to poor quality carbon fibers.

In addition, electricity consumption in commercial and residential buildings accounts for a significant portion of all electricity used in the United States. Building occupants can reduce building heating and cooling energy consumption by 10% if they can provide a sense of stability and reduce the building set-up to 4 ° C in winter and up to 4 ° C in summer. These savings account for more than 1% of the total energy consumed in the United States.

Thus, there is a need for a low energy method of stabilizing PAN fibers to produce uniformly stabilized fibers.

In addition, a fabric capable of generating heat through application of current is also required.

In one aspect, the disadvantages of the prior art are that the present invention, which is a method of producing carbon fibers by mixing carbon nanotubes (CNTs) into a solution comprising polyacrylonitrile (PAN) to form a CNT / PAN mixture Is overcome. One or more PAN / CNT fibers are formed from the mixture. The predetermined first current is applied to the PAN / CNT fiber until the PAN / CNT fiber becomes the stabilized PAN / CNT fiber.

In another aspect, the present invention is a heatable fabric comprising a plurality of fibers each having an axis. The plurality of fibers each comprise polyacrylonitrile and carbon nanotubes dispersed in a predetermined weight percent thereof in the polyacrylonitrile and aligned mostly along the axis of the plurality of fibers. The plurality of fibers are woven into the fabric. The source of current is configured to apply current through the plurality of fibers, thereby causing the fibers to generate heat.

These and other aspects of the present invention will become apparent from the following description of the preferred embodiments taken in conjunction with the following drawings. As will be apparent to those skilled in the art, numerous changes and modifications of the present invention may be made without departing from the spirit and scope of the novel concept of the disclosure.

Figure 1 is a flow chart illustrating one method of making carbonized fibers.
2 is a schematic diagram showing the current applied to the PAN / CNT fiber.
Figure 3 is a graph of the electrical conductivity in PAN / CNT fibers vs. the current in the fibers.
Figure 4 is a photomicrograph of a fabric comprising PAN / CNT fibers.

Preferred embodiments of the present invention are described in detail below. According to the figures, like numbers refer to like parts throughout the drawings. Unless specifically stated otherwise in the following disclosure, the drawings are not necessarily drawn to scale. As used in the description of the present application and throughout the claims, the following terms take the express meaning associated herein unless the context clearly dictates otherwise: "a "," an & Includes plural forms, and the meaning of "in" includes "in" and "on ".

As shown in FIG. 1, one embodiment is a method 100 for producing carbon fibers, wherein a solution is formed (110) by dissolving polyacrylonitrile (PAN) in a first solvent. A plurality of carbon nanotubes (CNTs) are suspended in a second solvent to form a suspension (112). The first solvent and the second solvent may comprise the same material. The suspension is added to the solution and the resulting blend is mixed to disperse (114) the CNT into the PAN solution. The fibers are formed 116 from the resulting blend (e.g., through a synthetic fiber spinning process, such as solution spinning or gel spinning). The fibers are drawn 118 at the desired diameter and a first current is applied to the fibers to stabilize the fibers 120 (i.e., to make the molecules of the fibers into a ladder structure). A second current is applied to the fibers to carbonize (122) the fibers.

A stabilization schematic is shown in FIG. 2, where the fibers 210 comprise a polymer matrix (e.g., PAN matrix) 212 and a plurality, mostly aligned CNTs 214 dispersed therethrough. A current 220 from a current source is applied to stabilize the fibers 210.

In one experimental embodiment, polyacrylonitrile (PAN) / CNT conjugate fibers having a CNT content of 15 wt% and 20 wt% were prepared using a dry-jet-wet spin technique. Carbon nanotubes (CNTs) exhibit electrical conductivity and can introduce current into the polymer. The electrical conductivity of PAN / CNT fibers is improved by annealing processes at different temperatures and changes with the passage of time. These fibers can also react to stretching, and when the elongation reaches 3%, the electrical conductivity is reduced to 50%. In addition, the current can induce a Joule heating effect and thermally deform the PAN / CNT composite fiber. By applying various currents up to 7 mA at fixed lengths, the conductivity was improved from about 25 S / m to over 800 S / m, and the composite fibers stabilized in the atmosphere. The temperature of the composite fibers may increase from room temperature to several hundreds of degrees Celsius as measured by infrared (IR) microscopy. The joule heating effect can also be estimated according to a one-dimensional steady-state heat transfer equation, which indicates that the temperature can be high enough to stabilize or carbonize the fiber.

In an experimental embodiment, polyacrylonitrile (PAN, molecular weight: 105 g / mol) with 6.7% methyl acrylate as the copolymer (obtained from Japan Exlan Co.) was dried at 80 캜 under vacuum before use. Carbon nanotubes (multi-walled carbon nanotubes) were obtained from Iljin Nanotech Co. of Korea. The PAN powder was dissolved in dimethylformamide (DMF, obtained from Sigma-Aldrich Co.) using an impeller at 90 DEG C and the CNT powder was dispersed in DMF using an ultrasonic bath (Branson, 3510-MT) . The CNT / DMF dispersion was then mixed with the PAN solution for fiber spinning and the PAN / CNT composite fibers were applied to a wet wet spinning unit (Bradford) with a spinneret diameter of 250 mu m and two coagulation (DMF / water) Obtained from University Research (England)). The fibers were then drawn from boiling water and dried in an oven at 50 < 0 > C for 7 days.

Joule heating effects were induced by applying current using a source measurement unit (Keithley 2400 Sourcemeter). The fiber structure was measured by a real time wide angle x-ray apparatus during the joule heating process. Wide angle x-ray diffraction (WAXD) using CuK _? (? = 0.1542 nm) was performed with an x-ray generator (Rigaku Micromax-002) with a 45 KV operating voltage and 0.65 mA current. The diffraction pattern was recorded by a detection system (Rigaku R-axis IV ++) and analyzed by AreaMax (version 1.00), and MDI Jade (version 9.0). From the WAXD data, PAN crystallinity, PAN crystal size, and Herman bearing parameters of polymers and carbon nanotubes were calculated.

Conductivities of PAN / CNT conjugated fibers having a 15 and 20 wt% CNT content were measured by 4-probe method. Conductivity of the composite fiber before the annealing process was approximately 10 ^-5 S / m. The CNT electrical conductivity is in the range of 10 ⁵ to 10 ⁶ S / m and the CNT content is up to 20 wt.% In this fiber, but a significant Schottky barrier between adjacent tubes can seriously reduce the conductivity and the effective CNT network CNT alignment was necessary for The electrical conductivity was significantly improved using an annealing process and the conductivity was 4.83 S / m and 27.63 S / m, respectively, for fibers with 15 and 20 wt% CNT content, after annealing at 180 ° C for 2 hours . Electrical conductivity increased with increasing annealing temperature.

The response of the conductivity to the annealing process was observed using a power meter and a temperature controlled oven. The composite fiber having a 20 wt% CNT content was adjusted at 180 ° C and a 10 μA current was applied. The voltage response at the start was rapid. After only one minute, the applied voltage was reduced to 40% and the electrical conductivity increased to approximately 2 S / m. After 2 hours of annealing, the voltage decreased to 95% and the conductivity reached 25 S / m.

To investigate the structural changes due to annealing, the composite fibers before and after annealing at 180 DEG C for 2 hours were also observed using x-ray diffraction. The structure of PAN changed after the annealing process, and the crystallinity and crystal size of the PAN polymer increased to 50-60% and 5.8-11.8 nm, respectively. The annealing process induced a recrystallization process of the polymer chains, resulting in higher crystallinity and larger crystal size. This induces rearrangement of the CNT network and lowers its orientation. The rearrangement increased the number of CNT inter-tube connections, thereby significantly improving the electrical conductivity.

After the annealing process, the response of the conductivity to the tensile strain was measured by an elongation test. The elongation at break was about 6% and about 3%, respectively, for composite fibers having 15 and 20 wt% CNTs. Both fibers exhibited a tensile strength higher than 130 MPa and a similar tensile modulus of about 9 GPa. During the elongation test, the electrical conductivity decreased with increasing elongation. Conductivity varied from 27 to 15 S / m for fibers with 20 wt% CNT at 3% elongation and 4.5 to 0.4 S / m for fibers with 15 wt% CNT at 6% elongation. This means that when the conjugate fiber is elongated, an elongation of only 3% can reduce the electrical conductivity by 50%. This phenomenon gives the expectation for polymer / CNT complexes and reduces the consequences of breakage in CNTs. During the stretching process, CNTs within the polymer matrix were oriented and higher alignment of CNTs lowered the possibility of tube-to-tube connections. Lower CNT connections lowered the electron transfer and resulted in lower conductivity. The response of electrical conductivity to strain also suggests that PAN / CNT composite fibers can be used as sensors to interact with the environment when external forces are applied to the fibers.

The electrical properties were measured when the applied current was less than 0.1 mA per filament (about 6 x 10 ⁴ A / m ² ) at a fixed length. When the applied current is higher than 1 mA per filament (about 6 x 10 ⁵ A / m ² ), the conductivity is significantly improved by increasing the current. As shown in FIG. 3, the conductivity of the annealed composite fibers with a 20 wt.% CNT content was originally about 25 S / m at a current of less than 1 mA. The electrical conductivity of the fibers increased with increasing applied current and reached 800 S / m at an applied current of 7 mA.

The effect of power on the composite fiber was further investigated using Fourier transform infrared spectroscopy (FT-IR). The spectral peaks at 3000-2850 cm ^-1 (CH, elongation), 2240 cm ^-1 (CN, elongation), and 1452 cm ^-1 (CH ₂ , bending) . However, a broad peak appeared at 1600 cm ^-1 (C = C or C = N, conjugate) where other peaks in the fingerprint region between 1600 and 500 cm ^-1 were C = C, C for a heteroaromatic ring = N, CO, -OH, and NH groups. Stabilization and comparison of composite fibers after joule heat treatment with carbonized fibers indicates that the composite fibers stabilized after the electric treatment at a fixed length. When the current passed through the fiber, the temperature of the composite fiber was increased by the joule heating effect. The temperature increased by the electric power was high enough to stabilize the composite fiber, so that the composite fiber was thermally deformed by the electric current.

Wide angle x-ray diffraction (WAXD) was also used to observe the joule heating effect on the composite fibers. When the different currents passed through the composite fibers having a 20 wt% CNT content, the two-dimensional WAXD pattern was recorded in real time. This diffraction pattern represents a visible change in the diffraction peak at about 17 ° and about 30 ° when the applied current is higher than 1 mA. These two peaks represent the PAN (200, 110) and (310, 020) crystal planes. If the current is higher than 1.6 mA, the diffraction peak disappears at about 17 ° and about 30 ° and the diffraction pattern is completely different from the original fiber pattern. This WAXD pattern demonstrates the proposal from FT-IR observation that the PAN polymer structure gradually changes with increasing current. The power induced the Joule heating effect, which thermally deforms the PAN crystal structure and stabilizes the composite fiber when the increase in the fiber temperature by electricity is sufficiently high to destroy the PAN crystal structure.

This structure was stable at low applied currents, only slightly increasing the crystallinity and orientation of the PAN polymer. However, when the current was higher than 1 mA, the PAN crystallinity and orientation decreased. After increasing the current to 1.6 mA, the crystallinity changed from 60% to 18% and the orientation factor decreased from 0.56 to 0.37. The d-spacing at 2θ of about 17 ° also changed from 0.525 to 0.539 and the crystal size increased from 11.8 nm to 16.6. This phenomenon also induced stabilized PAN fibers, but the PAN crystal structure was destroyed during the stabilization and carbonization process and transformed into a ladder or graphite structure.

The PAN crystal structure underwent a heat-strain process and the current was 1.6 mA, and one additional peak was found at 2θ of about 15 °. The variation of the WAXD pattern represents the deformation of the PAN crystal during the Joule heating process. This additional peak also suggests that the structural change can start from a portion of the PAN polymer region. Since the electrons pass mainly through the CNTs inside the polymer matrix, the joule heating process initially occurred around the CNTs, leading to a higher temperature region around the CNTs. Thus, thermal deformation of the PAN can start in the hot zone following the CNT and gradually occurs in the complete complex. Another reason is that PAN crystals of amorphous PAN and smaller crystals can have lower thermal stability and can be modified first, whereas PAN crystals of larger crystal size can be held for a longer time during joule heating. When the current is higher than 3 mA, all the PAN crystals are switched, and two peaks at 2? About 15 and about 17 disappear.

After the PAN structure disappeared at a current higher than 3 mA, the 26 ° peripheral peak became more dominant. This indicates that the stabilization of the composite fiber was induced by current and the PAN polymer was converted to a ladder structure. During the Joule heating process, the PAN structure was gradually converted with increasing current and the alignment factor of the stabilized ladder structure varied from 0.44 to 0.51, while the d-spacing and crystal size were almost the same at different applied currents. The electro-induced stabilization process suggests a new energy saving process for making carbon fibers.

The electrical properties of PAN / CNT composite fibers with 20 wt% CNT were also observed to investigate the effects of fiber length on the Joule heating pattern. Longer fibers require higher voltage to overcome higher electrical resistance. For example, fibers shorter than 4 mm require a voltage of less than 50 V only to reach a current of 1 mA, whereas fibers of length 76 mm require 500 V to reach a current of 1 mA. Since high voltage, as well as high power, could damage the fiber, the 76 mm length of the fiber resulted in only an applied current as high as 4 mA, and the fiber was broken at 4 mA. Since the required voltage and generated power are different for fibers of various lengths, when these fibers are applied as exothermic fibers, optimizing the fiber connection and weaving method is important to obtain a controllable and uniform temperature profile.

In an experimental embodiment, the electrical conductivity of the PAN / CNT composite fibers was investigated and the fibers with a 20 wt% CNT content exhibited a conductivity greater than about 800 S / m at about 25 S / m. Annealing the composite fibers can rearrange the fiber structure and improve the electrical conductivity as well as the CNT network. When the conjugate fiber is elongated, the electrical conductivity of the fiber can be a function of the elongation and can be reduced by only 50% by 3% elongation. In addition, the current induces a joule heating and thermally converts the CNT / PAN composite fiber applied with a current of 1 to 7 mA. The fiber temperature may gradually increase to 1000 ° C. As a result, the current can induce stabilization of the composite fibers in air.

In one embodiment, as shown in FIG. 4, the PAN / CNT fibers can be woven into a fabric 400 that can be heated by application of current from a current source 410. This embodiment relates to a polymer / carbon nanotube (CNT) fiber that can provide fibers, textiles, textiles, garments, and blankets that can be heated by application of very small amounts of electricity. These fibers are durable and provide textile quality (aesthetics, touch, etc ...). The use of these fibers is expected to provide a sense of security to the occupants, while setting the winter building temperature to be at least 5 ° C lower than the current setpoint temperature. It is expected to save about 0.4% of the total energy consumed in the US today, saving more than $ 1 billion annually.

In one experimental embodiment using gel spinning, polyacrylonitrile / carbon nanotube (PAN / CNT) conjugated fibers are made of CNTs that are well dispersed and mostly aligned along the fiber axis. CNT not only improves the mechanical properties but also can introduce electrical conductivity into the composite fibers. Depending on the CNT concentration as high as 20 wt.%, The PAN / CNT fibers exhibit a conductivity of approximately 25 S / m and the conductivity can be influenced by temperature, tensile strain, and electrical voltage. Therefore, the present inventors can introduce joule heat into the polymer / CNT fiber to realize active heating capability.

When the current was passed through the composite fiber, the Joule heating effect was significant for the PAN / CNT composite fiber. According to Joule's law and the one-dimensional steady-state estimated by the Poisson equation, the temperature of the PAN fiber containing 20 wt% CNT increases above 200 ° C when a current in the range of 1 to 7 mA is applied, It can be close. The electrical conductivity of the fiber increased considerably to almost 800 S / m when the current through the fiber increased from 1 mA to 7 mA. Appropriate temperature increases of fibers and fabrics can be achieved at much lower CNT concentrations and lower current levels.

This approach can be applied to a wide variety of substrates such as PAN and polypropylene (PP), poly (ethylene terephthalate) (PET), poly (ethylene) (PE), various nylons, poly (vinyl alcohol) (PMMA), poly (ethylene oxide) (PEO), poly (ether ketone) (PEK), polycarbonate (PC), and rubber. Depending on the proper process design and power, the composite fiber material can generate heat under control, which can lead to significant energy savings by creating high quality durable fibers, fabrics, textiles, garments and blankets with controlled heating capacity.

The above-described embodiments are provided as merely exemplary embodiments, including preferred embodiments and best mode of the present invention known to the inventors at the time of filing. It will be readily appreciated that there can be many variations from the specific embodiments disclosed herein without departing from the spirit and scope of the invention. Accordingly, the scope of the present invention should be determined by the following claims rather than being limited to the specifically described embodiments.

Claims (14)

(a) mixing a carbon nanotube (CNT) with a solution comprising polyacrylonitrile (PAN) to form a CNT / PAN mixture;
(b) forming at least one PAN / CNT fiber from the mixture;
(c) applying a predetermined first current to the PAN / CNT fibers until the PAN / CNT fibers become stabilized PAN / CNT fibers; And
(d) applying a predetermined second current to the stabilized PAN / CNT fibers until the stabilized PAN / CNT fibers are carbonized to form carbon fibers
≪ / RTI > 2. The method of claim 1, wherein the mixing step comprises
(a) dissolving PAN in a first solvent to form a solution;
(b) dispersing CNT in a second solvent to form a suspension; And
(c) adding the suspension to the solution
≪ / RTI > 2. The method of claim 1, wherein the predetermined first current comprises 1 mA to 5 mA current. 2. The method of claim 1, wherein the forming comprises spinning the mixture to form fibers. 5. The method of claim 4, wherein the spinning step comprises a gel spinning step. The method of claim 1, wherein the polyacrylonitrile is a copolymer; Homopolymers; ≪ / RTI > and combinations thereof. The method of claim 1, wherein the PAN / CNT fibers have axes and the carbon nanotubes are mostly aligned along the PAN / CNT fiber axis. The method according to claim 1, wherein the carbon nanotubes comprise up to 20% by weight of PAN / CNT fibers. The method according to claim 1, wherein the carbon nanotubes comprise 20 wt% or less of fibers. (a) a plurality of fibers each having an axis, wherein the plurality of fibers comprises a polymer matrix and carbon nanotubes dispersed in a predetermined weight percent in the polymer matrix and aligned substantially along the axis of the plurality of fibers, ; And
(b) a current source configured to apply current through the plurality of fibers to generate heat in the fibers;
≪ / RTI > 11. The composition of claim 10 wherein the polymer matrix is selected from the group consisting of polyacrylonitrile (PAN), poly (propylene) (PP), poly (ethylene terephthalate) (PET), poly (PVA), poly (methyl methacrylate) (PMMA), poly (ethylene oxide) (PEO), poly (ether ketone) (PEK), polycarbonate Wherein the polymer comprises a polymer selected from the list of polymers. 11. The exothermic fabric of claim 10, wherein the carbon nanotubes comprise a plurality of fibers of 20 weight percent or less. 11. The composition of claim 10, wherein the polymer matrix comprises a PAN copolymer; PAN homopolymer; ≪ / RTI > and combinations thereof. 11. The exothermic fabric of claim 10, wherein the plurality of fibers comprises carbonized fibers.

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